The definition of death is surprisingly slippery. Before the 1950s someone was usually considered dead if their heart had stopped pumping and they’d ceased to breathe on their own. But in 1968, a Harvard Medical School committee upended this definition, arguing that people were only dead when the brain showed no electrical activity as measured by an electroencephalogram (EEG).

For the next 50 years this concept of “brain death” has dominated medical definitions of death. When your brain stops functioning, you die. Simple. But what if you could start it working again?

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On April 17, researchers at Yale University in New Haven, Connecticut, raised precisely this question when they published details of a study showing it was possible to bring back limited cellular function in pig brains after slaughter. The results of the study were published in the scientific journalNature.

The group, led by neuroscientist, Nenad Sestan were able to restore functioning in tissue obtained from 32 pig brains, four hours after a loss of blood circulation and eventual brain death in the pigs which formerly housed them. The revival was accomplished by circulating an oxygen- and nutrient-rich solution through the brains using a complex system called BrainEx, specifically designed for the task.

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Up to six hours after the fluids were initially pumped, blood vessels and capillaries throughout the brains remained dilated, delivering oxygen and nutrients that were taken up by the surrounding tissue. The brains even showed levels of oxygen intake and carbon dioxide release comparable to a functional, intact brain. At this level of revival, these brains not only demonstrated less decomposition than other pig brains that were not hooked up to the BrainEx system, they began to show signs of recovery. They even engaged in a release of immune-system responses upon being activated to do so, suggesting the onset of self-reparation processes.

What’s more is that some synaptic activity – the basic unit of transfer of information in the brain – was recorded in some brain cells. When tissue was removed, neurons demonstrated a restored ability to function in terms of signal transfer.

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The findings enable in-depth study of brain function and structure not previously achievable. The technique can be used to gain new insights into how the brain is wired and can advance clinical methods of preventing brain damage caused by lack of blood flow that occurs during cardiac arrest or stroke, for example.

But did these disembodied pig brains regain consciousness?

Before the experimental testing began, the researchers considered this possibility, and therefore the chance that the pig brains would experience pain. To prevent such an occurrence they monitored activity and prepared cooling protocols and anaesthesia to subdue any activity deemed to possibly indicate consciousness.

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The clear answer, however, is no – the activations recorded in the partially revived brains were not comparable with recordings obtained from intact, aware brains. Bioethicist and co-author of the study, Stephen Letham said, “That monitoring didn’t show any kind of organised global electrical activity.” The pig brains met established criteria for brain death when they were obtained, during the study and after.

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Yet it is the first study to demonstrate that cellular function can be artificially restored in brain tissue after clinical brain death. Since its release, it has drawn much attention both in philosophical and ethical communities by raising questions that confuse current definitions of life and death. Additionally, it has ignited concerns regarding prioritization of resuscitation at the cost of organ donation.

A primary question regards how consciousness would be detected in a disembodied brain to begin with. If we are unable to detect any physical behaviour associated with consciousness it is impossible to attribute awareness to an organ without a body. Similarly, recordings obtained from brain monitoring techniques like electroencephalogram (EEG) can be compared with those obtained during conscious awareness but do not spell out consciousness in any yes/no dichotomy. Arguably, consciousness itself does not have a clear definition.

Ultimately, the study emphasizes that the line between life and death is blurred rather than clear – an issue that has come up time and again, for centuries.

In April 2013, 13-year-old Jahi McMath was declared brain dead after a botched surgery, yet kept on life support after showing signs of responsiveness to external stimuli. Her condition became a matter of serious legal, ethical and philosophical consideration as she continued to show signs of active behaviour and even aging all the while monitoring equipment showed little signs of meaningful brain activity. She was kept on life support until she died in June, 2018 – nearly four and a half years later.

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More similar to the current study, however, are a series of investigations conducted in the 1960s at Case Western Reserve in Cleveland that explored the ability to suspend and preserve brain function, using deep hypothermic cooling as low as -40C. Headed by physician, Robert “Humble Bob” White, the controversial experiments involved extracting and preserving monkey brains under cold conditions, which were then kept “alive,” functioning for hours or days using ‘donor’ monkeys to maintain cardiovascular activity.

In an unsettling ordeal, scientists, the media and ethicists questioned whether consciousness was retained in these brains, while the animals had no body to house them. Yet he believed his research could eventually make head transplants on humans with spinal injuries a possibility, using Stephen Hawking as an example candidate.

Experiments involving eschewing the ethical and philosophical lines between life and death, however, go back as far as the 18th century to the work of Italian physician and philosopher, Luigi Galvani (1737 -1798) and later, his nephew Giovanni Aldini (1762-1834). Their unsettling experiments convincingly showed the power of neural, electrical influence on muscle movements. They would, for example, electrically elicit facial movements in the severed heads of various animals including horses. Aldini went so far as to attempt to electrically reanimate the corpse of an executed prisoner, preserved in the cold of winter. This widely publicized feat drew a large crowd, generating tremendous controversy. ‘Galvanism’ as it became called, would later motivate author Mary Shelley to pen Frankenstein.

Thus, many of the issues raised by ‘Franken-pig’ brains are not new but rather more relevant than ever before, as our ability to restore life continues to advance. For Sestan, this now means starting to contend with increasingly tricky ethical questions. “An important next step is to create an ethics advisory body which should issue baseline guidelines for whether and how to conduct research if signals associated with consciousness were to emerge,” he says.